The present invention relates to a coherent optical multichannel (CMC) receiver and particularly to channel selection in CMC receivers.
CMC systems are known per se and one such system is described by P. W. Hooijmans, M. T. Tomesen and P. P. G. Mols, "A coherent multi-bitrate multichannel system for simultaneous transmission of 140 Mb/s TV and 560 Mb/s HDTV signals" Proceedings of 16th European Conference on Optical Communication (ECOC), Amsterdam, 1990, paper WeG2.2. CMC systems make use of tunable lasers for selection of one of the many transmitter optical frequency division multiplexed (FDM) channels. The principle is essentially the same as the currently broadcast TV signals. In a CMC receiver the wavelength/frequency accuracy of the lasers used as local oscillators is one of the most critical features in these systems. Under ideal circumstances, this accuracy is of the order of several hundreds of MHz. The channel selection preset accuracy of the intermediate frequency (IF), that is, the frequency difference of the optical frequencies of the received signal and local oscillator, is therefore of the same order.
Another critical feature associated with channel selection in a CMC receiver is the phenomenon of false locks. This will be described further with reference FIGS. 1A and 1B, 2 and 3 of the accompanying drawings which show respectively a known FSK dual filter CMC-receiver, the pass bands of the dual filters, the automatic frequency control (AFC) frequency detection characteristic for an unmodulated carrier signal with the automatic gain control (AGC) off and the AFC/frequency detection characteristic for a modulated FSK signal with the AGC on. The CMC receiver shown FIG. 1A comprises an optical mixer 10 for frequency down-converting a received FSK signal having a bit rate of 140 Mb/s and a frequency deviation between the tones of 1200 MHz. A distributed feedback laser local oscillator 12 is connected to the mixer 10. The laser local oscillator 12 is tuned by a control current applied to its input 14. The control current comprises an AFC signal formed by a combination of a channel selection preset tuning current applied to a terminal 16 which is summed in a summer 18 with a current derived from a data output 20 of the receiver.
The IF signal derived from the mixer 10 is applied to a gain control amplifier 22 having a control input 24. The gain stabilised IF signal is then applied to two frequency detection paths 26,28 whose outputs are connected to a subtracting stage 30, the output of which forms the data output 20. An AGC signal is derived by connecting a baseband peak detector 32 to the output 20. The output of the peak detector 32 is connected to the control input 24. The low frequency and dc components present in the data output signal are supplied to an integrator 34 which has a second input for a dc offset voltage V.sub.ofs. The purpose of the offset voltage will be discussed later.
The frequency detection paths 26,28 comprise bandpass filters 36,38 and square law detectors 40,42, respectively. The filters 36,38 have pass bands f0,f1, respectively, centered on different frequencies as shown FIG. 1B. The width of the passbands is the same. At any one time, the modulated signal will have only one of the two tones and accordingly only one of the detection paths will be active and providing an output to the subtracting stage 30. However when there is no modulation the outputs of both paths will be substantially equal amounts of noise which will be cancelled by the subtractor 30.
When the receiver has to select a channel, the AFC loop is interrupted and the local oscillator tuning current is preset to the required value, which value is stored within a memory (not shown). The channel selector (not shown) initiates a small frequency sweep of the local oscillator around the preset value. The AGC voltage will detect the presence of an IF signal whereafter the frequency control will be taken over by the AFC.
The frequency detection characteristic shown in FIG. 2 is mirrored about the f=0 axis due to the fact that the local oscillator 12 can be both at the left and right side of the received signal. It is desirable for the filters 36,38 to have identical transfer and noise characteristics. The frequency detection characteristic shown in FIG. 2 is used both for demodulation, that is IF-to-baseband conversion, and AFC. Referring now to FIG. 3, stable AFC locks can only occur in zero crossings of the detector characteristic under modulation with a positive slope. However with a small negative offset in the AFC loop it is possible to obtain a second stable zero at f.sub.FL (FIG. 2), while only one lock at f.sub.IF is allowed. In practice these two frequencies may be several GHz apart. However under operational conditions it is very difficult to determine if the receiver is locked at the required frequency.
In the circuit shown FIG. 1A it will be assumed that in the absence of IF, the AGC will cause the amplifier 22 to give maximum IF amplification. In this situation noise within the bandwidths of both IF filters 36,38 will be detected as squared IF (or N.times.N) noise by the frequency detector. If both filters 36,38 have identical characteristics, both detected outputs will be equal and hence give a zero output after subtraction. As a result the AFC will have no input which may cause the local oscillator to drift so that the receiver is possibly tuned to a false lock.
One method for avoiding false locks is to increase V.sub.ofs so that a detector characteristic of the type shown in FIG. 3 is obtained. Since there is only one positive slope zero crossing, false locks are avoided. However the effect of increasing V.sub.ofs is to shift the zero crossing point from f.sub.IF to f'.sub.IF. This shift in the frequency of the zero crossing is undesirable because it could lead to errors in the data being detected.